Computer dosen't boot up..

Hi,

When i switch on the pc both the power & hard disk lights come on(in the cpu tower)..However the hard disk light goes off in about 5 seconds...
Meanwhile the monitor powers on, but goes back to stand by mode...Hence no booting up..nothing..just the "stand by" monitor...can hear the fans working inside...
What could the problem be?The hard disk or the video card??

Thanks!!!